1. Field of the Invention
This invention is in the field of a method and apparatus for drying and grinding particulate material, including a pair of grinding means, one being of the type which produces a relatively coarse product and the other producing a relatively fine product. A common conduit directs the ground products from both the grinding means into a sifting means, and the fraction recovered by sifting is then passed to a metering means which selectively passes controlled quantity fractions back into either the coarse grinding or the fine grinding means.
2. Description of the Prior Art
In prior art grinding-drying installations, it is known to provide, for example, a hammer mill in combination with a riser or uptake through which heating gas flows to an air current or air swept separator or sifter. The latter removes the gravel which is then passed for subsequent comminution to the desired final fineness in a second comminution step. As an example of such a prior art system, we refer to German Pat. No. 1,439,835 which shows a grinding-drying installation with a feed pipe or uptake through which the heating gas flows, and utilizes two different types of mills which are attached laterally to the pipe at their discharge ends.
In this known grinding-drying installation there is the feature that the inlets of both mills are in selectively connectible connection through a branch point in the discharge pipe of the gravel coming from the sifter in such a way that the grinding may be carried out to completion either in one or the other mill. A special embodiment of a coal grinding installation using a tube mill and a hammer mill is disclosed, for example, in German Pat. No. 698,311. This type of installation uses a rotatable valve at a crossing point of the conduits coming from the raw material charger and the sifter with the conduits going onto the hammer mill and to a tube mill. In this way, the raw material may be conducted to the tube mill and the gravel to the hammer mill or the raw material and the gravel together may be directed to the tube mill or they may be directed together to the hammer mill.
Various known grinding-drying installations have used pre-comminution devices such as hammer mills which knock the material to be comminuted through a grate. The material falling out of the grate is conveyed through a riser or uptake to a sifter in which a separation takes place into finished material and material which is comminuted further. Such an arrangement is satisfactory where the material is very conducive to comminution, and there are no special requirements present as to the fineness of the grains and the uniformity of the final material.
The present invention is particularly directed to a special application of the drying-grinding technique wherein a starting material with a relatively high moisture content must be ground in a uniform manner to provide a fineness which provides as large a reaction surface as possible. In addition to the improved grinding result, the economy of the grinding-drying process must be satisfactory.